The Inheritance of Retinal Degenerations

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INHERITANCE PATTERNS

By tracing the pattern of affected and unaffected family members, it is often possible to determine if the retinal degeneration in a family is dominant or recessive and whether the mutated gene lies on an autosome or on a sex chromosome. An inheritance pattern can be determined only when more than one member of a family has a retinal degeneration. Once a pattern has been identified, genetic counseling should be provided to any family member who is at risk for having the disease or who is at risk for having children with the disease. In most cases, RP and other retinal degenerative disease are inherited in either an autosomal dominant, an autosomal recessive or an X linked pattern. Another unusual inheritance pattern, known as digenic, has been seen in a small number of families affected by RP.

Autosomal Dominant

In an autosomal dominant retinal degenerative disease, an affected person has one altered gene (the mutated gene) paired with one normal gene. These genes lie on one of the 22 pairs of autosomal chromosomes. About 20 percent of all families with RP have autosomal dominant RP.

When a person with an autosomal dominant retinal degeneration (whether male or female) and an unaffected partner have children there is 1 chance in 2, or a 50 percen chance, that the affected parent will pass on the altered gene, and therefore the disease, to each child. There is also chance in 2 that he (or she) will pass on the normal gene to each child. The unaffected partner will pass on one of his or her norm~ genes (Figure 5). A child who does not have an altered gene does not have the disease and cannot pass the gene (and the disease) on to his or her future children.

Figure 5, Autosomal Dominant
Figure 5. Autosomal dominant retinal degeneration
Affected Chromosome Chromosome with retinal degeneration gene Unaffected Chromosome Chromosome with normal gene


Most often, families with an autosomal dominant retinal degeneration can trace the disease back through several generations. Rarely, in some of these families, the disease seems to have skipped one or more generations. In some families, it is possible for members to inherit a gene for autosomal dominant retinal degeneration but not develop the disease. This gene is said to have reduced penetrance, which means that the gene's effect is somehow modified, or reduced, and sometimes does not cause retinal degeneration even when present. However, in other families, the disease in affected members may be so mild that there is no, or very few, symptoms. This gene is said to have variable expressivity, which means that the severity of the effect of the altered gene can vary from one affected family member to another.

The age at which family members first show symptoms of retinal degeneration may also vary. Some family members can experience symptoms of the condition early in life, possibly in childhood, whereas other affected members in the same family may not have symptoms until they are well into middle age. When a family member dies before symptoms of retinal degeneration are discovered, but has affected children, it often seems that the disease has skipped a generation. Any person at risk for a retinal degeneration should have a thorough examination by an ophthalmologist to detect mild or late onset cases of the disease.


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Date last modified December 4, 1998